The invention described herein is directed generally to comb filters, and particularly to a filter and method for separating the luminance and chroma components of a television signal encoded according to PAL standards.
It is well known that so-called "recursive" filters are well suited for reducing noise in repetitive signals such as television signals whose information content does not vary to a great extent from frame to frame. An example of one such filter is described in U.S. Pat. No. 4,064,530. An improved filter for television signals is described in U.S. Application Ser. No. 041,455, filed May 22, 1979, and entitled NOISE FILTER.
Common to most recursive filters for television signals is the fact that an incoming television signal is compared to a previously received television signal which has been delayed for one frame interval, for example. Depending on the differences between the incoming signal and the delayed signal, the incoming signal is altered to reduce its noise content.
In the case where the television signal has a chroma component, the filter must take into account the phase change which the chroma component undergoes from frame to frame. That is, two successive frames of a color television signal cannot be properly compared because the phases of their chroma components are not the same. To compensate for this problem, the chroma component of the delayed signal is separated from its luminance component, the chroma component is phase corrected, and then the luminance and chroma component are recombined. The delayed signal is then in proper form for comparison with the incoming television signal.
A chroma phase corrector and separator (comb filter) which achieves the functions set forth above for NTSC type television signals is disclosed in the above-mentioned patent application. However, where the filter is designed for use with television signals encoded according to the European
system, a different type of comb filter is required, due to the fact that the phase of the chroma component of the PAL signal varies from line to line in a more complex manner than does the NTSC signal.
Moreover, most television noise filter techniques now operate on a digitized television signal which has been sampled, as opposed to operating on an analog signal. Hence, it is necessary to employ a digital comb filter in the overall filter system. However, previous filters for
type television signals have been analog and/or extremely complex. There is need for a satisfactory digital type comb filter for processing
type television signals.